Transformers filled with dielectric liquid and working in medium and/or high voltage grids incorporate protective systems whose function is to eliminate the effects of various failures and to disconnect power from the grid if there is an internal short circuit in the transformer. The protective systems contain current-limiting fuses with tripping devices which control the disconnector and which are coupled with a control sensor used to control the pressure and level of oil inside the transformer tank. Exceeding the predetermined parameters of oil level or pressure results in shorting of fuses and consequently in the disconnection of the transformer. In known solutions for protecting transformers against internal faults, the disconnector whose movable contacts are situated on a rotary strip, contains current-limiting fuses fixed to the rotary strip and suitably spaced in one row, the spacing resulting from the dimensions of the external insulators in which the current-limiting fuses are located.
A disconnector for transformers filled with dielectric liquid intended for use in systems protecting the operation of electric power equipment is known from patent description EP 2282322. The disconnector comprises at least two cylindrical current-limiting fuses which are located inside the transformer tank. Each fuse is electrically connected with the external phase power supply and through fixed contacts and moving contacts of the disconnector with the active part of the transformer. The current-limiting fuses are located in a common casing which also houses a slide with a pilot. The moving contacts of the disconnector are permanently fixed to do the slide and they move together with the slide when the slide makes a to-and-fro motion. The to-and-fro motion is caused by the action of a tripping device located in the current-limiting fuses and by compressing or stretching springs attached to the slide pilot and to the fixing disc which is provided with a slide guide. The disconnector is activated if a fuse is blown or if the indication of the oil level sensor or the pressure sensor exceeds the safe threshold value defined for the given transformer. In a situation where the transformer is connected in parallel on the secondary circuit side i.e. on the low voltage side with another electric unit, the action of the disconnector mounted on the high voltage side does not guarantee that current will stop flowing in the damaged circuits of the transformer. The value of current flowing in the damaged circuits of the transformer may be lower than the detection threshold of the disconnector mounted on the high voltage side, which may result in a build-up of the effects of the failure until the transformer explodes.
From patent application DE1065515 there is known a combination of a disconnection fuse and a low voltage disconnection switch. In the presented solution the striker pins of the fuse mechanically actuate a low voltage switch used to switch off low voltage side.